I always use Verbatim DVD's for everything. For wii single layer backups I've always used -r, and never had a problem. For double layer backups you need to use +r, because you have to set the layer break, you cannot do so with -r discs.

Verbatim were always rated best back in the day but the difference nowadays is pretty slim, wii's don't mind a lot of half decent media. Cheap shit they may have issues but any half tidy brand should see you good